This is an odd issue, as I own the game through Steam, and I'm running it via Steam. Anyone having this same issue? I've verified the game files a few times now with no dice, and searching for this issue only brings up results for pirated copies.

Game's running fine here.
Have you tried to reinstall the game? Is so, go to your Steam folder and delete everything except the steamaaps folder and the steam.exe
Run the Steam.exe and wait for it to update...